1. Field of the Invention
The present invention relates to an improved method for cutting a hard and brittle material having a crystal structure, such as a silicon ingot, with a wire saw, and more particularly relates to a method for cutting a hard and brittle material with which variance in the warpage, nanotopography, and thickness of sliced wafers is reduced at all locations in the lengthwise direction of the ingot, and particularly at the ends, when the material is affixed to a fixing jig and sliced after its crystal orientation has been measured.
2. Description of the Related Art
Wire saw cutting devices, which use moving wires to simultaneously slice a large number of wafers from a columnar silicon monocrystalline ingot, have been widely used to manufacture wafers, which are semiconductor substrate materials.
FIG. 1 illustrates an example of the structure of a wire saw cutting device used for silicon monocrystalline ingots. As shown, in order to cut a large number of wafers at the same time from a silicon monocrystalline ingot, a wire 4 is wound around the outer periphery of three long rollers 1, 2, and 3 disposed in parallel, with the wire strands parallel and at a specific spacing, and the wire 4 played out from one wire bobbin 5 goes around the outer periphery of these rollers 1, 2, and 3 and then is wound up on another wire bobbin 6.
A silicon monocrystalline ingot 7 is affixed to a jig 8 at the place where the wires 4 are moving in the same direction and arranged at a specific spacing in the axial direction between the upper two rollers 2 and 3, and this jig 8 is lowered while being mechanically supported by a separate supporting mechanism (not shown), which presses the ingot against the wires 4 and cuts the ingot. In another configuration, the monocrystalline ingot 7 is pressed against the wires by being raised, rather than lowered.
Slicing with a wire saw is performed as above, but with a hard and brittle material having a crystal structure, such as a silicon monocrystalline ingot, because of the need to specify the crystal plane of the sliced wafers, the crystal orientation of the ingot 7 (the workpiece) with respect to the wire saw must be adjusted after the ingot is affixed to the jig 8. An orientation adjusting mechanism, which adjusts the crystal orientation by rotating the ingot 7 along with the jig 8, is therefore provided to the supporting mechanism that supports the jig 8, and this mechanism is used to adjust the crystal orientation.
Since productivity is diminished by providing an orientation adjusting mechanism to the support mechanism of the jig 8 as above, one approach that has been taken is to measure the crystal orientation of an ingot ahead of time, and affix the fixing jig 8 in the lengthwise direction at the required location of the outer peripheral surface of the material so that the ingot will be facing in the required direction during slicing (see Japanese Laid-Open Patent Application H11-77663).
In any case, as the diameter of a silicon monocrystalline ingot being sliced with a wire saw increases, there is a tendency for the warpage of wafers at the center of the ingot to be consistent, and for the warpage of wafers to worsen toward the ends of the ingot.
Also, Japanese Laid-Open Patent Application 2001-18219 discusses as a working example a method in which a silicon disk with a thickness of 20 mm and a diameter of 76 mm is used as an anti-warping member that is bonded with an adhesive to the end faces of a piece of silicon with a diameter of 76 mm and a thickness of 10 mm, this sample is disposed so that the center axis of the sample is aligned with the center axis of a primary coil and so that the primary coil and the sample are not in electrical contact with copper wiring, a voltage of 20 kV is applied to a capacitor, and after this charging, a switch is closed to generate a pulse magnetic field in the primary coil, and the above-mentioned silicon with a thickness of 10 mm is sliced.
With this method, which involves pulse magnetic field cutting using a conductor, either a member that prevents warpage is bonded to the ends of the material being cut, or the cutting is performed with this member pressed against the material, the primary object being to prevent buckling of the material being cut. With this cutting method, however, when a large number of wafers are cut from a long, slender ingot, it is impossible to reduce variance in the nanotopography and thickness of the wafers.
It is an object of the present invention to provide a method for slicing a hard and brittle material having a crystal structure, such as a silicon ingot, and more particularly a hard and brittle material cutting method which solves the problem of worsening variance in thickness, nanotopography, and wafer warpage.
As a result of close scrutiny of what an ingot undergoes during slicing, in an effort to reduce the warpage of wafers sliced from the ends of the ingot, the inventors perfected the present invention upon discovering that when retainer plates are bonded to or pressed against the ends of an ingot, and simultaneous slicing with a wire saw is performed along with the retainer plates, a portion of increasing variance in the warpage, nanotopography, and thickness will appear in the portions corresponding to the retainer plates, resulting in a decrease in variance in wafer warpage, nanotopography, and thickness at the ends of the targeted ingot.
Specifically, the present invention is a method for cutting a hard and brittle material, comprising the steps of:
measuring the lengthwise and peripheral crystal orientation of a columnar hard and brittle material having a crystal structure;
fixing a jig plate in the lengthwise direction of the outer peripheral surface of said material;
setting the tilt angle of the crystal plane of said material;
bonding or pressing a retainer plate on one or both end faces of said material; and
moving a wire saw relatively from the outer peripheral surface that is on the unrestrained side and bound to the jig, toward the jig, and thereby slicing the material into a large number of disk-shaped wafers.
The present invention is also a cutting method comprising the above steps, wherein the retainer plate is a disk, ring, or perforated disk of substantially the same diameter as the columnar hard and brittle material, or wherein the retainer plate is composed of either the same material as the columnar hard and brittle material, or of glass, ceramic, carbon, or resin, or wherein the pressing means presses a disk, ring, or perforated disk against said end face with a plurality of pins.